Hydrotalcite, which is a species of naturally occurring layered clay mineral, contains, as the main skeleton, hydroxides of elements such as magnesium, aluminum, etc. that naturally exist in abundance, and it is relatively easy to synthesize hydrotalcite. Hence, various synthesis methods have been disclosed. For example, one of the disclosed methods produces hydrotalcite in a water solvent by using magnesium hydroxide as a magnesium source (e.g., see Patent Literature 1), and another of the disclosed methods reacts magnesium ions with aluminum ions in an aqueous solution in the presence of alkali (e.g., see Patent Literature 2).
It is also known that hydrotalcite has an anion exchange effect. If hydrotalcite can work the anion exchange effect to immobilize arsenic, fluorine, boron, selenium, hexavalent chromium, nitrite ion, and other anionic hazardous substances, it can be expected to contribute to the technologies for improving safety of waste products or the technologies for detoxifying and reforming the environment, by improving the quality of contaminated water, preventing elution of hazardous substances, ameliorating soil, promoting stabilization of hazardous substances at waste disposal sites, etc.
One method of immobilizing anionic hazardous substances may mix powder hydrotalcite in a solution, in which hazardous substances dissolve, to immobilize the hazardous substances, and then isolate hydrotalcite from the solution by pressurization using a filter press or the like. However, this method requires a pressurization means for pressing hydrotalcite and the solution to a filter fabric with a large pressure, which leads to a problem that the whole instrument becomes large.
Hence, small facilities use granular hydrotalcite, which is a state of hydrotalcite solidified to form grains to enable anionic hazardous substances to be immobilized with a relatively small pressure. Granular hydrotalcite needs to have a large grain size so as not to cause filter clogging, etc., and conceivable methods for production thereof may use a granulating machine, a binder, etc.
Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. H6-329410
Patent Literature 2: Unexamined Japanese Patent Application KOKAI Publication No. 2003-26418